Electron donor-acceptor dyads based on ruthenium(II) bipyridine and terpyridine complexes bound to naphthalenediimide

Two series of photosensitizer-electron acceptor complexes have been synthesized and fully characterized: ruthenium(II) tris(bipyridine) {[Ru^II(bpy)₂(bpy-X-NDI)], where X = -CH₂-, tolylene, or phenylene, bpy is 2,2′-bipyridine, and NDI is naphthalenediimide} and ruthenium(II) bis(terpyridine) {[Ru^II(Y-tpy)(tpy-X-NDI)], where Y = H or tolyl and X = tolylene or phenylene, and tpy = 2,2′:6′,2″-terpyridine}. The complexes have been studied by cyclic and differential pulse voltammetry and by steady state and time-resolved absorption and emission techniques. Rates for forward and backward electron transfer have been investigated, following photoexcitation of the ruthenium(II) polypyridine moiety. The terpyridine complexes were only marginally affected by the linked diimide unit, and no electron transfer was observed. In the bipyridine complexes we achieved efficient charge separation. For the complexes containing a phenyl link between the ruthenium(II) and diimide moieties, our results suggest a biphasic forward electron-transfer reaction, in which 20% of the charge-separated state was formed via population of the naphthalenediimide triplet state.